Characterization of hepatic flavin monooxygenase from the marine teleost turbot (Scophthalmus maximus L.).

1. The presence and properties of flavin monooxygenase (FMO) in liver of the marine teleost, turbot (Scophthalmus maximus) were examined in relation to organic xenobiotic metabolism and osmoregulation. 2. Hepatic microsomes of sexually mature fish contained NADPH-dependent FMO as evidenced by the conversion of N,N-dimethylaniline (DMA) to DMA-N-oxide, and immunorecognition of single bands (approximate apparent molecular weight of 55 kDa) by antibodies to mammalian FMO 1 and FMO 2. Additionally, Northern analysis using a full-length cDNA probe to mammalian FMO 1 revealed a single hybridizing band of approximately 2.5 kb. 3. No significant differences were seen between male and female turbot FMO with respect to DMA N-oxidase activity, levels of immunoreactive protein (with anti-FMO 1 or anti-FMO 2) and gene expression (hybridizing mRNA). 4. Hepatic microsomal DMA N-oxidase activity was inhibited by methimazole (an FMO substrate) and trimethylamine (TMA), but not by piperonyl butoxide (a P450 inhibitor). Inhibition by TMA is indicative of a role for FMO in osmoregulation, catalysing the conversion of TMA to TMA N-oxide. DMA N-oxidase activity was optimal at pH 8.8 and 25 degrees C, and displayed Michaelis-Menten kinetics with respect to DMA (apparent Km = 88 microM).